1. Field of the Invention
The present invention generally relates to variable frequency oscillators, and more particularly to a variable frequency oscillator which varies the oscillation frequency based on a control voltage or control data.
Recent computer systems are required to transfer signals at a high speed and improve accuracy and reliability, and are further required to reduce the cost of hardware. In order to meet the above requirements, it is required to efficiently develop and design the systems and circuit configurations.
2. Description of the Related Art
FIG. 1 is a block diagram of a conventional voltage-controlled oscillator (VCO), which is an analog circuit. Inverters 10.sub.1 -10.sub.5 form a ring oscillator. Capacitors C1 and C2 for delaying signals are connected to the output terminals of the inverters 10.sub.2 and 10.sub.3, which are supplied with a frequency control voltage via a terminal 12. The frequency of an output signal at a terminal 14 can be varied by changing the signal delay times of the inverters 10.sub.2 and 10.sub.3.
However, the conventional voltage-controlled oscillator has a disadvantage in that the frequency of the oscillation signal depends on the number of stages of inverters and a wiring delay. Hence, the lower oscillation frequency is obtained by increasing the number of stages of inverters and the length of the signal line. This increases the circuit size. Further, although the voltage-controlled oscillators have an identical circuit configuration, they have different oscillation frequencies due to certain factors of the production process.
There is another disadvantage due to the arrangement in which the capacitance values of the inverters 10.sub.2 and 10.sub.3 are respectively changed in response to the frequency control voltage and thus the signal delay time can be changed. Thus, a large delay time causes an increased rounding of signal waves. In this case, if noise is superimposed to the signal at the time of the rising and/or falling edge of the signal waveform, the inverter which receives the above signal is liable to malfunction. The frequency of the output oscillation signal is liable to disperse over a certain range (jitter). A complicated circuit design and an increased circuit size will be required to reduce or cut noise form peripheral circuits.